Basic spilling implementation.

Add a spilling pass which lowers register pressure by assigning SSA
values to the stack. Important missing features:

- Resolve conflicts where an instruction uses the same value more than
  once in incompatible ways.
- Deal with EBB arguments.

Fix bugs in the reload pass exposed by the first test case:

- Create live ranges for temporary registers.
- Set encodings on created spill and fill instructions.

lib/cretonne/src/regalloc/mod.rs

@@ -14,6 +14,7 @@ mod diversion;
 mod pressure;
 mod reload;
 mod solver;
+mod spilling;
 
 pub use self::allocatable_set::AllocatableSet;
 pub use self::context::Context;